This invention relates generally to electric motors and, more particularly, to a method and system for selectively operating an electric motor in a first pole configuration or a second pole configuration.
Pole-changing motors have been used to supply multiple speeds in washing machines. These motors have two sets of windings for different pole structures wound into one stator. Typical induction pole-changing motors are limited to speed ratios of about 1.5:1 (4/6 pole motors), 2:1 (2/4 pole motors) or, at very costly penalties, 8:1 (2/16 pole motors). These motors typically have a significant amount of winding and stator material since only one set of windings is energized at a given speed while the other winding set remains de-energized or idle. Further, placing two pole winding sets within the stator necessitates very large stator slots, which results in inefficient use of steel material within the stator core. Finally, this construction results in a motor capable of operating at a limited number of discrete speeds.
Variable speed induction motors and electronically commutated motors wherein a motor speed is controlled electronically to vary the frequency of the power supplied to the motor are practically limited to a speed ratio of about 10:1. The low speed of the variable speed motor or the electronically commutated motor is limited by the loss of motor efficiency and speed control while operating at frequencies below 30 Hz, while the high speed is typically limited by the impedance, stator losses of the motor and/or computational speed of the control at frequencies above 300 Hz.
Universal motors have also been used to achieve variable speed with a high-speed ratio. This construction, however, has significant reliability limitations due to the commutators and brushes required to excite the armature. Additionally, universal motors produce a commutation noise that is undesirable.